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Thin people may be able to summon more mental defenses to resist tempting, high-calorie foods than obese people, researchers said on Monday.

Brain scans of thin people who looked at pictures of high-calorie foods showed increased activity in a region of the brain used for impulse control, but obese people showed little activity in this region, the researchers found.

"I think there essentially may be biological reasons why people can't necessarily control their desire for food," said Robert Sherwin of Yale University School of Medicine in Connecticut, who worked on the study published in the Journal of Clinical Investigation.

The study is part of a push to understand the underlying biological processes that contribute to obesity, which affects more than one third of adults and nearly 17 percent of children in the United States.

Researchers at Yale and the University of Southern California used functional magnetic resonance imaging, or fMRI, to examine areas of the brain that become active when a person views images of high-calorie foods, healthy foods like fruits and vegetables and non-food items.

The study involved 14 healthy people, nine thin and five obese volunteers, who underwent brain scans two hours after eating. The researchers manipulated blood sugar levels, testing the subjects when they had normal and low blood sugar levels.

They found that when blood sugar levels were low, brain regions called the insula and striatum associated with rewards are activated, signaling a desire to eat.

The prefrontal cortex, which normally dampens signals to eat, was less able to put the brakes on signals generated from the striatum to eat.

That was especially true in the obese study subjects who were shown pictures of high-calorie foods.

But when blood sugar levels were normal, the thin study subjects showed greater activity in the prefrontal cortex, and this reduced activity in brain regions involved in rewards.

"There is a controller, a higher function that controls your reward centers. That controller is deficient in people with obesity. They don't activate that system," Sherwin said in a telephone interview.

He said larger studies are needed to confirm the findings, but the study does suggest that obese people may be less able to shut off parts of the brain that drive food cravings.